QUESTIONS.

1

How many primary planets are there in the so

lar system ?

How many secondary planets are there?

How do the primary planets perform their revolutions round the Sun ?

In what time does Mercury revolve round the Sun?

In what time does Venus revolve round the Sun, and how far is she at her mean distance from that luminary?

In what time does the Earth perform its revolution round the Sun, &c. ?

In what time does Mars complete his revolution, &c.

In what time does Vesta complete a revolution, &c. ?

In what time does Juno complete her revolu

tion, &c. ?

In what time does Uranus perform his revolution, &c. ?

How do the secondary planets perform their revolutions?

Name the circumstances respecting the motions, &c. of the Comets.

Before we conclude this chapter, it may not be improper to make some observations on the solar system; and also to

describe the different systems which have been invented, in order to explain the natural appearances of the heavenly motions.

Obs. 1. The most celebrated systems of the world are the Ptolemaic, the Copernican or Pythagorean, Pyth and the

Tychonic.

2. The Ptolemaic System, so called from the celebrated Ptolemy, an Egyptian philosopher, who flourished at Alexandria about 130 years after the Christian era.

In this system, he supposed with the vulgar, who measure every thing by their own conceptions, that the earth was fixed immoveably in the centre of the universe; and that the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn, revolve round it in the order of distances in which they are mentioned. Above these he placed the firmament of the fixed stars, the crystaline orbs, the primum mobile, and last of all, the cœlum empyrium, or heaven of heavens. All these vast orbs were supposed to move round the earth once in twenty-four hours, and also in certain stated or periodical times, agreeably to their annual changes and appearances. Every star was supposed to be fixed in a solid transparent sphere, like crystal; and, to account for their different motions, he was obliged to conceive a number of circles called eccentrics and epicycles, which crossed and intersected each other in various directions. And if any new motion was discovered, a new heaven of crystal was formed to account for it. So that, as Fontenelle observes, heavens of crystal cost him nothing, and he multiplied them without end, to answer every purpose.

Although this system was supported by many of the old philosophers, and, indeed, almost all astronomers, for nearly 1400 years; yet it has long since been rejected by the most eminent mathematicians and philosophers.

It is now well known that the planets, Mercury and Venus, do not include the earth in their orbits; because, if the earth were the centre of motion, they would be sometimes in opposition to the sun, which is never known to be the case. Besides, the comets moving through the heavens in all manner of directions, must infallibly have met with continual obstructions, and would, long ere this, have broken these crystal spheres to pieces, and rendered them totally unfit for the purposes for which they were designed.

3. The Copernican, or Solar System, which is now universally adopted by all mathematicians and astronomers, is not only the true, but also the oldest system in the world. It was

first of all, as far as we know, introduced into Greece and Italy, about 500 years before Christ, by Pythagoras; from whom it was called the Pythagorean system. But, from the accounts of his disciples and followers, it is evident, that it was not the result of his own observations, but that he had received hints of it from some more enlightened nations, who had made greater advances in the science of astronomy. It is most probable, indeed, that the doctrine was transplanted by him from the east, in which part of the world he spent two and twenty years, and scrupled not to comply with all the customs peculiar to the eastern nations, in order to obtain free access to their priests and magi, to whom al most all knowledge of the arts and sciences was then con fined. And as he was a man of extraordinary qualities, and had an insatiable thirst for knowledge, so he seems to be the most successful of any of the ancients in making himself acquainted with their philosophies.

The Pythagorean system had, in a great measure, been lost during several ages; but Copernicus, a bold and original genius, retrieved it about the year 1500; from whom it took the name of the Copernican System.

Copernicus having adopted the Pythagorean or true system of the universe, published it to the world with new and demonstrative arguments in its favour, in his work entitled De Revolutionibus Orbium Cælestium, first printed at Nuremberg in 1543, a little previous to his death, which took place in the same year, at the age of seventy. In this treatise he restored the ancient Pythagorean system, and deduced the appearances of the celestial motions from it in the most satisfactory manner. Every age since has produced new arguments in its favour; and notwithstanding the opposition it met with from the prejudices of sense against the earth's motion, the authority of Aristotle in the schools, &c.; the truth of the ancient Pythagorean system, by applying mathematical reasoning to mechanical experiments, was established by Sir Isaac Newton; and upon this foundation he raised the superstructure of that philosophy, which, whilst all other systems sink into ruins, and little more than their inventors' names are remembered, will remain for ever firm and unshaken; for being once demonstrated to be true, it must eternally remain to be so, as nothing can alter it but the utter subversion of the laws of nature, and the constitution of things.

In the ancient Pythagorean system, which was revived by

Copernicus, clearly demonstrated by Newton, and which is now adopted by all astronomers and mathematicians, as the true system of the universe; the sun is placed in the centre, about which the planets revolve, from west to east, in the following order of distances; Mercury, Venus, the Earth, Mars, Jupiter, Saturn, and Uranus; beyond which, at an immense distance, are placed the fixed stars. The Moon revolves round the earth; and the earth revolves about its own axis. The other secondary planets move round their respective primaries from west to east at different distances, and in different periodical times.

4. Although the Copernican System was received by most men of science then living, yet there were some who would never assent to it. The motion of the earth was so contrary to what they were always accustomed to hear on the subject, and, as they thought, to appearances, they could never agree to support such doctrine. Among those who opposed the system of Copernicus, was Tycho Brahe, a Danish nobleman, who was born in 1546, and who devoted the whole of his life to the study of astronomy. As Tycho could not entirely adopt the Ptolemaic system, being convinced that the earth is not the centre about which the planets revolve, and out of respect for some passages of Scripture, which seemed to contradict the doctrine of the Pythagorean system, which Copernicus had lately revived; he invented a new system, which was a kind of mean between the Ptolemaic and Copernican. In the Tychonic System, the Earth is placed in the centre of the orbits of the Sun and Moon; but the Sun is supposed to be the centre of the orbits of the five primary planets then known.

In this new system of Tycho, there is some ingenuity, though but little conformity to truth and observation. For having rejected the diurnal rotation of the earth on its axis, he was obliged to retain the most absurd part of the Ptolemaic hypothesis, by supposing that the whole universe, to its farthest visible limits, was carried by the primum mobile about the axis of the earth continually every day. But in this, however, he was abandoned by some of his followers, who chose rather, to save this immense labour to the spheres, by ascribing a diurnal motion to the earth; on which account they were distinguished by the name of Semi-Tychonics.

5. Besides these different systems, there is also another, called the System of Des Cartes, which, on account of its being the celebrated system of vortices, may not be improperly taken notice of here.

In this system of Des Cartes, the Sun is supposed to be placed in the centre of a vast whirlpool of subtle matter, which extends to the utmost limits of the system, and the planets, being plunged into such parts of this vortex as are equal in density with themselves, are continually dragged along with, and carried round their several orbits by its con stant circulation. Those planets which have satellites are likewise the centres of other smaller whirlpools which swim in the great one; and the bodies that are placed in them, are driven round their primaries, in the same manner as those primaries are driven round the sun. Now as the sun turns on his axis the same way that the planets move round him, and the planets also turn round their axis the same way as their satellites move round them; it was imagined, that if the whole planetary region was filled with a fluid matter, like that before mentioned, the sun and planets, by a constant and rapid motion on their axis, would communicate a circular motion to every part of this medium, and by that means drag along the bodies that swim in it, and give them the same circumvolution.

This, in a few words, is the celebrated system of vortices, and the world of Des Cartes. The fabric, it must be confessed, is raised with great art and ingenuity, and is evidently the produce of a lively fancy and a fertile imagination. But then it can be considered only as a philosophical romance, which amuses without instructing us, and serves principally to show that the most shining abilities are frequently misemployed; and will always be found inadequate to the arduous task of forming a complete system of nature, which is not to be expected even from the labour of ages.

Besides various objections which may be brought against Des Cartes' system, it has been demonstrated by Newton and others, that let the nature of these vortices be what it may, yet the circulations, in such a fluid, would never agree with the known laws of their motion, established by later astronomers, from repeated observations. But, admitting for a moment that this system of whirlpools was compatible with the phenomena of nature, and the laws of mechanics, still their cause would be but little better; for no such whirlpools have ever yet been shown to exist. It is not sufficient that a hypothesis accounts for the phenomena; but it must be shown that it is founded in fact, and sane tioned both by reason and experience.